The kinetic patterns of fresh, frozen-thawed, or cultured human leukocytes are studied by tagging the cells ex vivo with 111-Indium, a radioisotopic label, and measuring their distribution throughout the body by means of gamma camera imaging and gamma count-ing of blood samples. This type of study has widespread applications. The most common use of these radiolabeled leukocyte trafficking studies is for abscess localization in cases of sus-pected infection not definitively diagnosed by other noninvasive studies. In these cases, auto-logous or allogeneic granulocytes, collected by simple phlebotomy or by apheresis, are labeled with 50 uCi of 111-Indium per kg of patient weight, not to exceed 500 uCi total. Twenty-three studies localizing abscess were performed in Clinical Center patients enrolled in a variety of protocols in the past 8 years. Of these 23, three were positive and received appropriate therapy: for salmonella osteomyelitis in a hemophiliac patient with HIV infection, staphylococcal pneumonia in a patient with aplastic anemia, and diverticular abscess in a patient with Zollinger-Ellison syndrome. In many of the negative studies, unnecessary surgical exploration was avoided. Three of the 23 Indium-labeled allogeneic leukocyte studies were performed in patients with chronic granulomatous disease, in one case to document clearance of pulmonary infection, in the second to confirm lack of trafficking in an alloimmunized reci-pient, and in the third to localize a cause for fever. In all three cases, gamma imaging results were used to guide therapy, specifically to terminate a course of granulocyte transfusions when they were either no longer necessary or were ineffective or to prevent an exploratory laparotomy. Collaborative trials with the Surgery Branch, NCI, have investigated the diagnostic utility and prognostic application of radiolabeled autologous tumor infiltrating lymphocyte (TIL) studies in patients with metastatic melanoma. TIL trafficking studies revealed metastatic deposits that were undetected clinically, and TIL trafficking to sites of tumor was strongly correlated with tumor regression following TIL infusion. In contrast, studies of Indium-labeled autologous, cloned T cells with antimelanoma activity did not show any trafficking to tumor sites in 14 patients. The results of the trafficking studies were highly predictive of clinical response in that none of the 14 patients had regression of disease in response to the cloned antimelanoma T cells. On the basis of these studies, the protocol was terminated, and a modified approach using highly immunosuppressive conditioning prior to infusion of autologous cloned T cells will be tested. Trafficking of Indium-labeled autologous cells will be used as one of the benchmarks with which to evaluate the response to therapy.